PICOSECOND TIME-RESOLVED ABSORPTION DYNAMICS IN THE ARTIFICIAL BACTERIORHODOPSIN PIGMENT BR6.9

The picosecond molecular dynamics in an artificial bacteriorhodopsin (BR) pigment containing a structurally-modified retinal chromophore with a six-membered ring beginning at C-9 to bridge the C-9=C-10-C-11 region of the polyene chain (BR6.9) are measured by picosecond transient absorption (PTA). Time-dependent absorption intensity and spectral changes in the 560-670 nm region are monitored for delays as long as 54 ns after the 4-ps, 573-nm excitation of BR6.9. Within <50 ps, two intermediates are observed, both of which have enhanced absorption to the red (>600 nm) of the BR6.9 spectrum. The first intermediate decays with a time constant of 5 +/- 1 ps to form the second, but no other absorption changes are found during the remainder of the initial 54-ns period of the BR6.9 photoreaction. Since these PTA properties are generally analogous to those measured in the native BR photocycle for J-625 and K-590, the two BR6.9 intermediates are denoted J6.9 and K6.9, respectively. The low-power energy, resonance Raman (RR) spectrum of ground-state BR6.9 is significantly different from that of native BR-570, thereby confirming that these PTA data are assignable to BR6.9 and its photoreaction alone and not to native BR species (BR-570 could remain in the reconstituted sample as a contaminant). The C-C stretching band structure in the RR spectrum of BR6.9 is similar to that of another of the artificial BR pigments in which the six-membered ring is incorporated to bridge the C-11=C-12-C-13 bonds, namely BR6.11. Mechanistically, these results demonstrate that restricted motion in the C-9=C-10-C-11 region does not change that part of the BR6.9 photoreaction involved in forming J6.9 and K6.9 but does alter the rate at which the J to K transformation occurs. A molecular model correlating the primary events and their rates in the native BR photocycle with those appearing in the BR6.9 photoreaction, as well as with other artificial BR pigments containing carbon rings, is presented.